Forget about whether or not it's rude - it's not even correct to call a homeless person a "hobo".
In the depths of the Depression, the term "hobo" referred to someone who was hitching a ride in a box car or walking from town to town, in search of gainful employment. They weren't "homeless" - they had a home and family to go back to, although they often had no money in their pocket. They were simply looking for work, wherever they could find it.
During the Depression, my father and my uncle rode a box car from Texas to Florida, and then walked back home to Texas again, looking for work and working at whatever kind of job they could find along the way. You could have called them "hobos" during that era.